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Originally posted by apowers I looked in the manual that came with my evaporator coil. There is a table titled "Coil Static Pressure Reading (Dry Coil)". It has the following info for my model at 1600 cfm (doesn't show any other cfm value):

.23 in. w.c. for Dry
.29 in. w.c. for Wet

From what I understand, my unit runs at 1985 cfm at the maximum speed for cooling.

Your contractor took your ESP readings in the correct places as you stated in your opening paragraph, at the blower opening and at the top of the furnace.

What it looks like to me that might be confusing everyone is he just took it a step further and broke the supply side ESP down and the return side ESP down.

Your return ESP was -.32 on his meter.
Your supply side was + .67 on his meter.
This gave you a TESP of .99

Your coil is fine as noted by your referencing to your literature of a .29 W.C. wet coil. This is exactly what the contractor showed in his breakdown.

You need to lose about .26 from your supply side ESP which will get you down to .73 TESP and this will start to give your blower motor some relief.

Next you need to work on the return and try to gain .10 from it.

This will get you down to a TESP of .63 where your system could live and try to work.

I also noticed you stated you have a 5 ton system, yet the literature shows 1600 CFM. Is this a 4 ton coil you have?

Originally posted by apowers I looked in the manual that came with my evaporator coil. There is a table titled "Coil Static Pressure Reading (Dry Coil)". It has the following info for my model at 1600 cfm (doesn't show any other cfm value):

.23 in. w.c. for Dry
.29 in. w.c. for Wet

From what I understand, my unit runs at 1985 cfm at the maximum speed for cooling.

I know others have hinted at it but. Is it possible the contractor installed too big of a unit previously and the system is designed for smaller tonnage?. Seems likely. Maybe the system is acually a 4 ton that someone forgot to set the CFM correctly on the furnace I.E. 1600 is the actual CFM required not the 1985 your trying to get? Shoehorning an oversized unit in is common when a cusstomer wants a bigger unit (can't talk him out of it so give him what he wants). There is a misconception that variable speed units will 'push' the air through.

From what I remember about the static pressure readings, the contractor measured between the entry to the blower and the top of the furnace for the .99 value. In addition, he drilled a hole just above the evaporator coil, at the start of the supply air plenum and measured the .38 value. My furnace sits on a platform with the return air coming in from below. He drilled a hole through the platform just in front of the bottom of the furnace and media filter and measured the .07. I’m not sure if that clears up anything or not.

As far as the information I gave about the coil, here’s some more background. The house was originally built for A/C (architect’s drawings) but I did not buy it until it was about 20 years old. I do not remember the size of the old unit that I replaced in 1989. It might have been a 4 ton or a 5 ton. Just don’t know for sure. The unit I put in was a Lennox 5 ton system. I was fairly happy with it. I replaced it with another 5 ton Lennox system this spring.

The coil that I have now has a Lennox part number of CX34-50/60C-6F. This is the one that has a specification of 1600 cfm in the table. I was under the assumption that this was a 5 ton coil. I can see what you mean by wondering if it is from the data on the table. I had gotten bids from several contractors and the ones that specified a model number all put that one. The only CX34 coil that shows 2000 cfm in the table is the CX34-60D-6F or CX34-62D-6F. These models are 3 ˝” larger in the cabinet width and would probably be matched with a bigger furnace (not for sure about that). I live in Southeast Texas so really cold weather is not a big problem here.

One thing that happened on the system that I put in 1989 was that I had to replace the blower motor at 9 years of age and 13 years of age. The motors got really noisy and stopped working. After the motor was replaced each time, the service person would have to replace the blower wheel since it was no longer in balance with the new motor. Having the advantage of hindsight, I am beginning to think that a high static pressure might have caused an early failure of the motors.

Guessing, I think you have a 5 ton system on a 4 ton duct system. I see why the contractor wants to replace your ductwork. The ECM motor is running at max RPM to maintain the CFM on the restrictive supply ductwork/coil combo. An accurate assessment of your ductwork with an acurate heat load calculation (vs the replace with the same assessment done previously) might have led you down a different path.